In accordance with the general terminology utilized in the printing industry, the word "letterpress" refers to a printing procedure in which the locations on the printing plate where ink is to be deposited are raised with respect to areas where ink is not to be deposited. Within the general designation of letterpress printing, two distinguishable forms can be identified. The first typically utilizes a relatively stiff printing plate (i.e. employing a material of relatively high stiffness), commonly referred to in the industry as a "hard" plate. "Hard" plate letterpress systems typically employ an impression roll with a compliant coating and one or more form cylinder(s) also with compliant coating(s). The form cylinder may be either directly inked from a well, or remotely inked through a series of rollers. The ink on the form cylinder is transferred to the inking locations on the "hard" plate which is mounted to the plate cylinder. The web or sheet of substrate to be printed is entrained between the impression cylinder and the plate cylinder. With a "hard" plate, the impression cylinder must be relatively less stiff, in order to avoid damage due to mechanical interference, and/or to improve the evenness of ink transfer from the printing plate to the substrate to be printed.
In this specification, the quality of "stiffness" means the resistance of a material to deformation under a given force. For example, if equal thicknesses of two different materials were placed on a hard surface, and a given weight over a given area were impressed upon each of the materials in order to deform or "pinch" the material, the material with the greater stiffness would yield less than the material with the lesser stiffness.
The second letterpress category utilizes a printing plate (commonly referred to in the industry as a "soft" plate) whose stiffness is relatively lower, i.e. the raised areas which are to be inked and then transfer the ink to the substrate are relatively less stiff with respect to the relatively more stiff form cylinder(s) and relatively more stiff impression roll (frequently steel).
The term "flexographic" is often utilized to refer to the second letterpress system described immediately above, in which a less stiff plate is used e other two rollers being relatively more stiff.
In the raised plate printing method the printing plates are normally made with as uniform a total thickness as is possible.
The printing industry generally recognizes certain inherent problems relating to the raised plate printing method using a soft plate of uniform resilience. One of these problems relates to the degree to which the printing surface of the plate is urged against the substrate, depending upon the area of coverage of the ink. It is known that the degree to which a plate surface is urged against the substrate is preferably less for the less covered areas, and more for the more covered areas. The "urging" comes about due to the squeezing or pinching of the soft plate between the substrate and the plate cylinder. When the area less covered includes tiny dots due to the four-colour separation process, the dots are printed by an upstanding cone having on top a flat portion which accepts ink and prints the dot. It is found generally that the amount of plate squeezing necessary to properly print solid-ink areas is too great to allow correct printing of the dotted areas, because the conical support below the inked surface creates excessive contact pressure which in turn tends to expel ink from the space between the paper and the raised dot on the plate, thus forming a ring or doughnut of solid ink around a central zone of inadequate ink coverage. On the other hand, if the degree of squeezing between the plate and the impression roll is reduced to a level which allows a good printing of the dot, it is found that areas of solid ink are inadequately printed, i.e. the ink is not fully and/or properly transferred to the substrate.
It is known to provide, for use with a printing plate, a "make ready" plate which corresponds to the plate in the sense that the "make ready" plate has an increased thickness in the regions corresponding to the more solid ink printing, and a gradually decreasing thickness in proportion to the degree of ink coverage in other regions of the plate. Areas of low ink coverage will include locations where fine copy appears. The "make ready" is positioned under the plate with corresponding areas matched, so that all solid regions will tend to be urged more strongly against the substrate (i.e., squeezed more) than are the areas which are only partially ink covered. It is understood that this process works to some extent, but not fully. It involves considerable extra expense to fabricate the "make ready" sheet, and it complicates the process of affixing the plate to the plate cylinder.
Relative to the affixing of the plate to the plate cylinder, where a plate of relatively low stiffness is utilized without the "make ready plate", it is typical in the industry to use a sheet of two-sided adhesive tape between the plate and the cylinder. Such tape may be very compliant (referred to in the trade as "cushion tape"), incorporating a layer of open or closed cell foam which is usually very low in stiffness. It is also known to use relatively stiff or non-compliant tape. It has been found that, when a low-stiffness tape is used to secure the plate to the plate cylinder, the plate-to-substrate contact pressure drops off too greatly in the locations of high ink coverage (area-wise), while the contact pressure between plate and substrate in the locations of relatively low ink coverage (area-wise) tends to allow more acceptable printing as the dots become smaller. The low-ink coverage areas are referred to as the highlight areas of the four colour printing process. Conversely, when a stiff tape is used, the dot areas extrude ink outwardly to a larger diameter than originally intended, and the locations of heavy ink coverage (area-wise) usually print relatively properly.
Among the prior art known to the applicant, U.S. Pat. 3,103,168, issued Sep. 10, 1963 to Braznell et al, exemplifies the difficulties encountered when using a plate component with a varying thickness. Because of this variation in thickness, it tends to be difficult if not impossible to ensure that the plate achieves a proper "fit" around the cylinder. In the four-colour separation process, each combination of printing plate and make-ready, as taught by Braznell et al, will have a different configuration, with the raised parts varying in height (thickness) between the different colors. For example, if the picture to be reproduced has a lot of yellow, the yellow plate would be effectively thickened up substantially compared to the plate for a colour which is less in evidence than the yellow. This would certainly mean that the four printings would likely fail to coincide or "fit" together. Another problem is that of "register", which has to do with keeping the web at the right "repeat length" with respect to the plate cylinder. What happens is that, because the plate (the upper surface of the plate) is digging into the web, and into the impression roll if it is a soft one, the web is actually driven by the plate, particularly where the thickness of the plate is excessive. Thus the web can be (and very frequently is) forced out of proper registry by overly thick plate regions.
Another patent of some interest is U.S. Pat. No. 3,169,066, issued Feb. 9, 1965 to Hoerner. Hoerner describes a process for sensitizing a polymeric body such that exposure to light, through either a positive or negative of a picture, initiates either a selective softening process (to create areas that can be abraded away) or a hardening process (wherein the non-exposed areas can be abraded away). In particular, Hoerner describes the possibility of using transparent blocks for making a printing plate, the transparent blocks allowing the light to pass directly through from one surface to the other, thus producing a reverse image on the bottom surface. Hoerner refers to this bottom image briefly as a "make-ready". However, in actual fact Hoerner does not change the stiffness of the various regions or columns affected by the light, in such a way as to vary the stiffness per unit printing area. In other words, even though Horner provides, for each "cone" to print a dot of colour, a reverse "cone" on the other surface, the column of plastic between the upper conical shape and the lower conical shape is not stiffer or less stiff than the plastic material occupying a similar cylinder in a solid-ink region.
One of the prerequisites for the carrying out of the present invention is a polymer which, upon exposure to suitable radiation (such as light), will undergo graduated hardening or graduated softening. A patent of interest in this area is U.S. Pat No. 3,549,366, issued Dec. 22, 1970 to Margerum, the specification of which is hereby incorporated by reference. The Margerum patent discloses a method of effecting optical hardening of photosensitized acrylamide compositions. The patentee indicates that the images are initially illuminated and projected or cast upon the prepared photosensitive acrylamide composition by visible radiation, this being followed by uniform illumination of the composition with visible light, and subsequent uniform ultraviolet radiation hardening.
Another patent of interest is U.S. Pat. No. 3,137,633, issued Jun. 16, 1964 to Kline. Kline discloses the discovery that the density of cross-linking in polyethylene or silicones depends to some extent on the energy absorbed per gram. The disclosure of this patent is hereby incorporated by reference.
Yet another patent of interest is U.S. Pat. No. 4,790,919, issued Dec. 13, 1988 to Baylor, Jr. Although directed to the preparation of electrophoresis gel material, the patent does discuss the photoinitiation of polymerization, and further discusses varying the intensity of light in order to adjust the degree of polymerization. The disclosure of this patent is hereby incorporated herein by reference.
A further patent of interest is U.S. Pat. No. 4,557,994, issued Dec. 10, 1985 to Nagano et al, the specification of which hereby incorporated herein by reference. This patent discloses a printing plate which is light-sensitive, and lists a number of suitable materials.
A still further patent of interest is U.S. 3,798,035, issued Mar. 19, 1974 to Varga et al, the disclosure of which is hereby incorporated herein by reference. Varga et al teach that the extent of crosslinking in a photopolymer is dependent on the combination of intensity and duration of radiation. This is inferred from the admission that the extent of cross-linking diminishes as depth from the exposed surface increases, and it is clear that the intensity of the radiation would decrease with increasing distance from the exposed surface.
A further patent of interest is U.S. Pat. No. 3,874,376, issued Apr. 1, 1975 to Dart et al, the disclosure of which is hereby incorporated herein by reference. Dart et al indicate that the degree of polymerization of a photopolymer depends upon the intensity of the visible light used.
In view of the foregoing discussion, it is an object of one aspect of this invention to facilitate optimum printing with a raised plate without having to vary the distance from the printing surface to the plate cylinder, wherein the urging of the plate surface against the substrate under the more solidly inked areas of the plate is greater than under the partially inked areas. Preferably, the urging varies continuously such that it is roughly proportional to the degree of ink coverage. Alternatively, the variation may be in discrete steps, again roughly proportional to the degree of ink coverage.
More particularly, this invention provides, for use in a raised image printing process employing a plate support, an improved plate construction comprising:
a flexible plate portion having an upper printing surface for printing an image on a substrate, the image including areas of greater ink coverage and areas of lesser ink coverage,
and means incorporated integrally into the plate portion for providing graduated regions of greater and less resistance to deformation below said upper printing surface, said means affecting the degree to which the surface is urged against the substrate with greater resistance to deformation occurring under image areas of greater ink coverage, and lesser resistance to deformation occurring under areas of lesser ink coverage, the plate portion having a uniform thickness, whereby said upper printing surface is uniformly spaced above the plate support.
Additionally, this invention provides a method of printing using a raised image printing process, the method comprising the steps:
providing a support surface;
providing a plate portion which has an upper printing surface for printing an image;
providing a support surface;
providing a plate portion which has an upper printing surface for printing an image;
securing said plate portion to said support surface such that said upper printing surface is at a uniform spacing from said support surface;
providing graduated regions of differing resistance to deformation integrally within said plate portion without altering said uniform spacing, such that a greater degree of resistance to deformation is provided under image areas of greater ink coverage, and a lesser degree of resistance to deformation is provided under image areas of less ink coverage; and
printing an image utilizing said upper printing surface.